The technique of multicarrier modulation, associated for example with a technique of error correction encoding and an interlacing operation, provides an efficient solution to the problem of information broadcasting and transmission, for example in a radiomobile environment. Thus, the COFDM (“Coded Orthogonal Frequency Division Multiplexing”) technique of modulation has been chosen for the DAB (“Digital Audio Broadcasting”), DVB-T (“Digital Video Broadcasting-Terrestrial”) and HIPERLAN/2 (“High Performance Local Area Network”) standards.
The multicarrier modulation used in the COFDM system, described for example in the French patent No. FR 2 765 757, comprises a particularly simple system of equalization, based on the insertion of a guard interval. This guard interval, also called a cyclic prefix, ensures that the system behaves well in the face of echoes, at the cost of a loss in spectral efficiency. It is in order to avert this loss, or at least to reduce it, that new multicarrier modulations are currently being studied. Among these, the invention relates especially to OFDM/OQAM (“Orthogonal Frequency Division Multiplexing/Offset Quadrature Amplitude Modulation”) for which the carriers are shaped by the Iota prototype function. It may be recalled that the Iota prototype function, described for example in the patent document No. FR 2 733 869, has the characteristic of being identical to its Fourier transform. The invention can be applied of course also to any other type of multicarrier modulation, especially the OFDM/OQAM type, whatever the associated prototype function.
The method used for shaping an electrical signal from the information to be transmitted depends of course on the conditions in which such a signal is transmitted. Here below, we briefly recall the characteristics of a transmission channel, especially in a radiomobile environment, for a clearer understanding of the value of the use of multicarrier modulations in such a channel.
In a radiomobile environment, the wave that is sent undergoes multiple reflections in its journey, and the receiver therefore receives a sum of delayed versions of the sent signal. Each of these versions is randomly attenuated and phase-shifted. This phenomenon, known as delay spread, generates inter-symbol interference (ISI). For example, in an urban type of environment, the delay spread is in the range of a few microseconds or less than a few microseconds.
Since the receiver (for example a motorist's mobile radiotelephone) is assumed to be in motion, the Doppler effect also acts on each path. This results in a frequency shift of the receiver spectrum, proportional to the speed of movement of the receiver. There also exist many other types of Doppler effects, all of which can be taken into account in the present invention.
The combination of these effects results in a non-stationary transmission channel, showing deep fading at certain frequencies (a frequency-selective channel is thus obtained). For certain applications, which are particularly useful in the context of the invention, the transmission band has a width greater than that of the coherence band of the channel (namely the band for which the frequency response of the channel may be considered to be constant over a given duration). Fading phenomena therefore appear in the band: i.e., at a given point in time, certain frequencies of the band are highly attenuated.
To overcome these different phenomena (due to the ISI and the Doppler effect), the addition of a guard interval was envisaged especially in OFDM type systems. The guard interval envisaged was one in which no payload information is transmitted, so as to ensure that all the information received comes from one and the same symbol. In the case of a coherent demodulation of the sub-carriers, the distortion contributed by the channel is then corrected by estimating its value at every point of the time-frequency network.
The introduction of such a guard interval of this kind reduces the problems related to inter-symbol interference, but one drawback of this prior art technique is that its spectral efficiency is reduced, since no information is transmitted during the guard interval period.
In the invention, therefore, a technique was sought to reduce the inter-symbol interference affecting the multiple carrier signals, without introducing any guard interval.
In order to provide for clearer understanding of the phenomena of interference between the symbols and/or between the carriers of a multiplex, the main characteristics of a multicarrier modulation are recalled here below. A multicarrier modulation is above all a digital modulation, namely a method for the generation of an electromagnetic signal, from a piece of digital information to be transmitted. The originality and value of such a modulation lies in the fact that it subdivides the frequency band allocated to the signal into a plurality of sub-bands, chosen so that their width is smaller than the coherence band of the channel, and on which the channel may therefore be considered to be constant for the duration of the transmission of a symbol. The digital information to be transmitted for this duration is then distributed to each of the sub-bands, so as to:                diminish the modulation speed (namely increase the symbol duration), without modifying the transmitted bit rate;        model the action of the channel on each of the sub-bands in a simple way, in making use of the model of the complex multiplier.        
At reception, a low-complexity system for the correction of the received data (in which a complex division is carried out by the estimated channel) is used to retrieve the information sent on each of the carriers, except for the carriers that have undergone deep fading. In this case, if no step is taken to protect the information, the data conveyed by these carriers will be lost. A multicarrier system is therefore useful only if the generation of the electrical signal is preceded by digital processing of the data, for example the application of an error correction code and/or an interlacing for example.
There are especially two known types of orthogonal multicarrier modulation. They are described for example in the patent document No FR 2 733 869, whose characteristics are recalled here below.
The whole set of carriers of a multicarrier modulation forms a multiplex. Each of the carriers of this multiplex is shaped by means of a same prototype function, referenced g(t), which characterizes the multicarrier modulation. The reference v0 denotes the spacing between two adjacent carriers of the multiplex, and τ0 denotes the temporal spacing between two multicarrier symbols sent. The signal sent, at each instant nτ0, on the mth center frequency sub-band νm, is αm,neiφm,ne2iπνmtg(t−nτ0), where the values αm,n represent the digital data to be transmitted. The expression of the signal sent in baseband (centered about the signal sent in baseband Nν0) is then:
                              s          ⁡                      (            t            )                          =                              ∑            n                                                          ⁢                                    ∑                              m                =                0                                                              2                  ⁢                  N                                -                1                                      ⁢                                          a                                  m                  ,                  n                                            ⁢                              ⅇ                                  ⅈ                  ⁢                                                                          ⁢                                      φ                                          m                      ,                      n                                                                                  ⁢                              ⅇ                                  2                  ⁢                                                                          ⁢                  ⅈ                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                                      mv                    0                                    ⁢                  t                                            ⁢                              g                ⁡                                  (                                      t                    -                                          n                      ⁢                                                                                          ⁢                                              τ                        0                                                                              )                                                                                        (        I        )            
It will be noted that, with the view to simplification, the case envisaged here is that of a signal having an even number of frequency sub-bands. It is of course more generally possible to write the signal in the form:
      s    ⁡          (      t      )        =            ∑      n                            ⁢                  ∑                  m          =          0                          M          -          1                    ⁢                        a                      m            ,            n                          ⁢                  ⅇ                      ⅈ            ⁢                                                  ⁢                          φ                              m                ,                n                                                    ⁢                  ⅇ                      2            ⁢                                                  ⁢            ⅈ            ⁢                                                  ⁢            π            ⁢                                                  ⁢                          mv              0                        ⁢            t                          ⁢                  g          ⁡                      (                          t              -                              n                ⁢                                                                  ⁢                                  τ                  0                                                      )                              where M represents the number of carriers of a reference symbol of the signal. It will indeed be recalled that, according to a standard technique, digital data αm,n with a value of zero are introduced into the edges of the spectrum, thus modifying the number of terms that effectively come into play in the above sum and, for example, make it possible to bring the number of carriers to an even number.
The functions gm,n(t)=eiφm,ne2iπmν0tg(t−nτ0) are called the time-frequency translated functions of g(t). To retrieve the information transmitted by each of the sub-carriers, it is necessary to choose g(t) and the phases φm,n so that the above << time-frequency >> translated functions are separable. A sufficient condition for verifying this property of separability is that these translated functions should be orthogonal in the sense of being a scalar product defined on all the functions of finite energy (which is a Hilbert space in the mathematical sense).
It may be recalled that the space of the finite energy functions accepts the following scalar products:                the complex scalar product        
  〈            x      ⁢                      y        〉              =                  ∫        R            ⁢                        x          ⁡                      (            t            )                          ⁢                              y            *                    ⁡                      (            t            )                          ⁢                  ⅆ          t                                    the real scalar product        
  〈            x      ⁢                                  y          〉                R              =                ⁢      e      ⁢                        ∫          R                ⁢                              x            ⁡                          (              t              )                                ⁢                                    y              *                        ⁡                          (              t              )                                ⁢                      ⅆ            t                              
Thus, two types of multicarrier modulation are defined:                a complex type of multicarrier modulation, for which the function g(t) chosen ensures an orthogonality, in the complex sense, of its translated functions. This is the case, for example, of OFDM, also called OFDM/QAM (“Orthogonal Frequency Division Multiplexing/Quadrature Amplitude Modulation”). For a modulation of this kind, φm,n=0 and the data αm,n are complex.        a real type of multicarrier modulation, for which the chosen function g(t) guarantees an orthogonality, in the real sense, of its translated values. This is the case, for example, with the OFDM/OQAM, OFDM/OMSK (Offset Minimum Shift Keying) or OFDM/OQAM/IOTA type modulations. For modulations of this type, φm,n=(π/2)*(m+n) and the data αm,n are real.        
The characteristics of these two types of modulation give rise to notable differences, especially in terms of density of the time-frequency network associated with the modulation considered.
It may be recalled that, since these multicarrier modulations are designed to transmit information, especially at high bit rates, their spectral efficiency is fairly high and may, for example, reach 4 bits/Hz (in digital television especially). The conversion of the bits coming from an error correction code into modulation symbols (this process is known as “mapping”) will thus be of the QAM (Quadrature Amplitude Modulation) type.
The transmission of a piece of complex data coming from the QAM constellation is therefore implemented differently depending on the type of multicarrier modulation used.
Thus, for a complex type of modulation, the real and imaginary parts of a piece of complex data coming from the QAM constellation are transmitted simultaneously, at every symbol period Ts. In the case of the real type of modulation, on the contrary, the real and imaginary parts are transmitted with a temporal shift of half a symbol period (Ts/2) (this is referred to then as the Offset QAM or OQAM).
For a same transmission band and a same number of sub-carriers, in order to transmit the information with the same bit rate, it is necessary, therefore, that the rate at which real-type multicarrier symbols are sent should be twice as fast as the rate for complex-type multicarrier symbols.
Furthermore, these two modes of information transmission are characterized by the density of the associated time-frequency network d=1/(ν0τ0). Thus the real-type multicarrier modulations correspond to a density d=2, while the complex-type multicarrier modulations correspond to a density d=1.
The distinct characteristics of the real-type multicarrier modulations, on the one hand, and of the complex-type multicarrier modulations, on the other hand, induce different processing operations during the implementation of an estimation of the transmission channel. In the case of a real-type multicarrier modulation, and as explained here below in this document, the channel estimation process is indeed made more difficult because the only orthogonality available for the translated functions is orthogonality in the real sense. To provide for clearer understanding of this problem, we shall now seek to describe a known channel estimation technique implemented in the context of a multicarrier modulation as presented here above.
It is assumed, in the reasoning developed here below, that the choice of the parameters of the multicarrier modulation ensures that the channel may be considered to be flat on each of the sub-carriers, for each OFDM symbol. The channel can then be modeled by a complex coefficient to be estimated, Hm,n (where m is the index of the sub-carrier and n is that of the OFDM symbol considered).
A classic technique used to estimate the channel in OFDM, consists of the insertion, into the stream of payload carriers, of reference carriers at positions known to the receiver. At reception, the values taken by these reference carriers, known as pilots, are read and the complex gain of the channel at these reference positions is easily deduced. Then, the complex gain of the channel at all the points of the time-frequency network transmitted is derived from the computed value of the complex gain at the reference positions.
In the OFDM/QAM context, a method was envisaged in particular, relying on an implementation of an estimation by reference multicarrier symbols (or preambles). According to this technique, at least one reference symbol is placed at the beginning of a frame, a frame being formed by a set comprising at least one reference symbol, called a preamble, and a set of payload symbols. Through this symbol or these symbols, the channel is estimated on each of the carriers of the multiplex. The choice of the parameters of the system (such as the symbol duration, frame length, etc) ensures that the channel varies slowly relative to symbol time. It is then assumed to be almost constant on a frame. The estimate of the channel on the reference symbols can therefore be used for all the OFDM symbols of the frame. This type of estimation is recommended in the HIPERLAN/2 standard (“Broadband Radio Access Networks (BRAN); HIPERLAN Type 2 Technical Specification; Physical (PHY) layer”, DTS/BRAN-0023003, October 1999).
The invention presented in this document can be applied more particularly to this method, known as the method of channel estimation by reference symbols.
As mentioned here above in the case of a multicarrier modulation of the OFDM/OQAM (Offset QAM) type, the channel estimation process is made more difficult because the only orthogonality available for the translated functions is orthogonality in the real sense. Indeed, to estimate the complex gain of the channel on a given sub-carrier, it is necessary to carry out the complex projection of the signal received on the sub-carrier considered. Now, the orthogonality of the translated functions in the real sense and the fact that the prototype functions, even when they are chosen to be localized to the utmost in time-and in frequency, have infinite support on at least one of the two axes, namely the time axis or the frequency axis, imply that, even on an ideal channel, there will be (intrinsic) interference between carriers.
Indeed, in the context of a real type of multicarrier modulation, the imaginary parts of the projection of the signal received on the basis of the translated values of the prototype function is not zero. A disturbance term then appears and gets added to the demodulator signal, and must be corrected before the estimation of the channel is carried out. It is therefore necessary to conceive methods that can be used to compensate for this loss of complex orthogonality and thus overcome the drawbacks of this prior art technique for OFDM(OQAM type modulations.
Indeed, according to the technique explained here above, the invention uses the complex projection of the multicarrier signal received r(t), at the point (m0,n0) of the time-frequency space to estimate the channel Ĥm0,n0 at this position. Thus if √{square root over (E)} is sent at (m0,n0), we have:
            H      ^                      m        0            ,              n        0              =            ∫                        r          ⁡                      (            t            )                          ⁢                              g                                          m                0                            ,                              n                0                                      *                    ⁡                      (            t            )                          ⁢                  ⅆ          t                            E      Assuming that the channel is ideal (r(t)=s(t)), we should therefore have: Ĥm0,n0=1.Now:
                              ∫                                    s              ⁡                              (                t                )                                      ⁢                                          g                                                      m                    0                                    ,                                      n                    0                                                  *                            ⁡                              (                t                )                                                    =                              E                    +                                                    ∑                                                      (                                          m                      ,                      n                                        )                                    ≠                                      (                                                                  m                        0                                            ,                                              n                        0                                                              )                                                                                                                ⁢                                                a                                      m                    ,                    n                                                  ⁢                                  ∫                                                                                    g                                                  m                          ,                          n                                                                    ⁡                                              (                        t                        )                                                              ⁢                                                                  g                                                                              m                            0                                                    ,                                                      n                            0                                                                          *                                            ⁡                                              (                        t                        )                                                                                                                                ︸                                                I                                                            m                      0                                        ,                                          n                      0                                                                      ∈                iR                                                                        (        II        )            
The equation (II) expresses the fact that the complex projection of the perfectly transmitted signal is nevertheless affected by an ISI (inter-symbol interference) intrinsic to the OFDM/OQAM modulations. The term “ISI” refers to interference between temporal symbols and/or between carriers.
The existence of this intrinsic ISI greatly disturbs the estimation of the transmission channel.
The invention is aimed especially at overcoming these drawbacks of the prior art.
More specifically, it is a goal of the invention to provide a technique of multicarrier modulation enabling the cancellation or, at least, the reduction of the intrinsic interference between symbols and/or between carriers.
It is another goal of the invention to implement a technique of multicarrier modulation that is simple and cost little to implement.
It is yet another goal of the invention to provide a technique of multicarrier modulation suited to OFDM/OQAM type systems.
It is also a goal of the invention to implement a technique of multicarrier modulation that can be used to adapt the method of the-channel estimation by reference symbols to OFDM/OQAM type signals.
It is also a goal of the invention to provide a technique of multicarrier modulation to implement a mode of channel estimation by reference symbols that is more precise than estimation using the prior art techniques.
It is yet another goal of the invention to implement a technique of multicarrier modulation enabling improved reception, demodulation and decoding of the multicarrier signal sent.
It is also a goal of the invention to provide a technique of multicarrier modulation for the cancellation or at least the reduction of intrinsic interference on whole OFDM symbols.